1. Field of the Invention
The present invention relates to an optical control element using a photonic crystal, and more particularly, relates to an optical control element that is able to be made compact, has high performance, and can be used in high speed/capacity optical communication and high speed optical signal processing in the field of optical transmission associated with a data transmission speed over 100 Gbps.
2. Description of the Related Art
When performing high speed and high capacity optical communication and high speed optical signal processing, optical signal pulses may spread, or the optical signals may arrive at different timings when being transmitted in a fiber, that is, dispersion or distortion occurs. This problem prevents high speed communication. To solve this problem, a device is needed for controlling the dispersion and a group speed delay of the optical pulses, which determine the optical signal arrival timing. In the related art, such devices have been developed, which utilize an optical fiber of peculiar dispersion properties. In this kind of device, light propagates in the optical fiber of peculiar dispersion property while the group delay is adjusted appropriately.
However, in the above device, a long optical fiber has to be used; for this reason, the optical control element becomes large in size, and has a low degree of freedom in the dispersion property. Due to this, it is difficult to reduce the size of the optical control device and increase the degree of integration, which are required for high level signal processing or multiple lines parallel processing.
Concerning dispersion compensation, study has been made to enable precise dispersion control and dispersion compensation by utilizing a fiber grating of a chirped structure. However, in order to achieve dispersion compensation, an optical fiber of an order of meters has to be used, hence it is difficult to reduce the size of the optical control device and increase the degree of integration. Further, because in a device including the chirped structure fiber grating, the incident light has to be reflected for use, and a circulator is needed to obtain high efficiency, hence, it is difficult to reduce the size of the optical control device and increase the degree of integration.
To solve these problems, recently and in the continuing years, dispersion or group speed delay effect given by a photonic crystal is attracting attention. In the photonic crystal, or in an optical wave guide with line defects introduced (usually referred to as a “defect wave guide”), the dispersion property, that is, a relation between the frequency and the wave number, shows some peculiarities.
For example, Japanese Laid-Open Patent Application No. 2000-121987 (referred to as “reference 1” hereinafter), and Japanese Laid-Open Patent Application No. 2000-224109 (referred to as “reference 2” hereinafter), disclose a dispersion compensation device using the photonic crystal.
However, in the dispersion compensation devices disclosed in reference 1 and reference 2, because the light propagating in the photonic crystal is not confined inside the wave guide structure, the dispersion compensation devices exhibit strong angular dependence, and have problems in reliability. Further, it is difficult to reduce the size of the dispersion compensation devices. For these reasons, these dispersion compensation devices disclosed in reference 1 and reference 2 cannot be put into practical use.
On the other hand, in the defect wave guide, it is theoretically predicted that the group speed of the light becomes zero at the edge of a Brillouin zone, which is also referred to as “band edge”, and in fact, a group speed as low as 1/90 of the speed of light in a vacuum has been observed at the band edge. However, the defect wave guide usually has very large wavelength dispersion, hence, although the group speed can be decreased when short pulses of light having definite spectral widths are input, because of the large wavelength dispersion, the pulses greatly expand. For this reason, it is difficult to obtain a device for dispersion control and group speed control by using such a simple defect wave guide.
On the other hand, in a structure referred to as a “coupled defect wave guide”, in which discrete dot-defects are arranged in sequence, because relatively large dispersion can be obtained in a relatively large area, and the value of the dispersion is higher than the dispersion of an optical fiber by about six orders of magnitude, the length of a fiber dispersion compensation device can be reduced from a few km due to the conventional techniques to a few mm.
For example, Japanese Laid-Open Patent Application No. 2002-333536 (referred to as “reference 3” hereinafter) discloses such a dispersion compensation device using the coupled defect wave guide.
FIGS. 40A through 40C are views of the conventional dispersion compensation device using the coupled defect wave guide as disclosed in reference 3.
As illustrated in FIGS. 40A through 40C, the dispersion compensation device includes a usual wave guide and a coupled-defect type dispersion compensation wave guide. Although it is structure-dependent, the dispersion compensation wave guide is calculated to be 20 ps/nm/mm.
However, in a slab type photonic crystal, which has been studied long before and is easy to be fabricated, if the coupled defect wave guide is formed, the period of the coupled defect wave guide in the propagation direction of the light is long, hence it is difficult to avoid a light leakage condition, known as “light-cone”, consequently, a large loss of light occurs. Due to this, the photonic crystal cannot be put into practical use at all.
On the other hand, in recent years and continuing, it is proposed to use a multi-layer one-dimensional periodic structure to render light to propagate in a direction perpendicular to the films of the multi-layer structure, so as to fabricate a dispersion control device under the same principle as the coupled defect wave guide. However, in this method, the light propagates in the space but not in a wave guide, so it is difficult to reduce the size of the device and increase the degree of integration.
In contrast, as described in the Extended Abstracts, the 64th Autumn Meeting, 2003, The Japan Society of Applied Physics, p 947, 1pZM14, and in the Extended Abstracts, the 50th Spring Meeting, 2003, The Japan Society of Applied Physics, p 1130, 28pYN1, inventors of the present invention have proposed a novel chirped photonic crystal, in which the diameter of holes in a line defect wave guide is arranged to change gradually. This structure is investigated theoretically and experimentally to demonstrate that at a specific propagation frequency, the group speed becomes zero at the band edge, and it is published that it is possible to perform dispersion control and group speed control.
However, when short light pulses are input, specific dispersive wavelength components are localized in some locations, such as the holes, and because of this, the waveform of reflected light pulses spreads or distorts.